1. Field of the Invention
The present invention relates in general to semiconductor devices, and relates in particular to a semiconductor device having a field-shield device isolation structure for isolation of devices in a high density MOSLSI circuit and a method of fabrication thereof.
2. Description of the Related Art
Isolation of device elements fabricated on silicon substrate has often been carried out by forming thick oxide films on selected local areas by the so-called Local Oxidation of Silicon method (shortened to LOCOS method hereinbelow). However, one of the serious barriers to achieving a higher density of integration by the LOCOS method has been a lateral growth of oxide film from the peripheries of the oxide film towards active device regions, the so-called bird's beak growth phenomenon which occurs during the thermal oxidation process. For this reason, there have been interests in other methods of isolation of active devices in large scale integration circuits, and particular interests have been focused on an isolation approach called field-shield device isolation method.
Field-shield device isolation method relates to a MOS (metal oxide semiconductor) structure for separating devices by forming a field-shield insulator film and a field-shield electrode between two active regions of a device. By fixing the field-shield electrode potential at a reference potential (e.g, GND or 0V), the formation of parasitic channels on the device surface is prevented, thereby providing insulative isolation of the active regions of the devices. In the presentation below, the field-shield device isolation is referred to as FSDI.
An improved version of the FSDI structure is known in which the field-shield insulator film and the field-shield electrode are buried in a trench. By utilizing such a buried-structure configuration, it is expected that higher integration of circuits may be realized.
However, the conventional FSDI structure is still inadequate for the purposes of attaining the degree of integration densities required in modern semiconductor devices. For example, even in those devices having the buried FSDI structure intending to increase the integration density, the top portion of T-shaped cross-section field-shield electrode is exposed on the surface of the substrate such that the width of the top end of the field-shield electrode is wider than the width of the trench width. In other words, much of the valuable surface space is occupied by the non-active regions serving the purpose of device isolation. This is obviously an undesirable aspect of the conventional FSDI structure, and there has been a need to improve the structural configuration of the existing FSDI stricture to meet the increasing demand for higher integration density in modern semiconductor devices.